The present invention relates to a method and apparatus for adapting the sensitivity of a sensor system for the detection of precipitations during the control of wiping intervals of a screenwiper drive of a motor vehicle as a function of the kinetic energy of striking precipitations detected by a pressure-sensitive sensor of a sensor system and converted into electrical output signals, with the output signals of the sensor being sent via a transmission path to an amplifier with an amplification factor for the purpose of activating a wiper-motor control circuit.
With a known automatic control of motor vehicle screenwiper systems, such as described in SAE Paper 85 1637, the wiping intervals are adjusted continuously between 0 and 20 seconds. The vehicle user can switch the automatic interval mode on and off by means of the screenwiper control switch. The sensor system of this control, received in a housing and arranged on the outer surface of a motor vehicle, consists of a vibratory plate, a piezoelectric element fastened to this vibratory plate and a signal amplifier.
The piezoelectric element converts the kinetic energy of precipitations striking the vibratory plate into pulse-shaped electrical voltage signals which are amplified. Noise fractions are filtered out, for example by means of a band-pass filter. The voltage pulses are finally fed to an integrating capacitor, the output of which is connected to an input of a voltage comparator. When the output voltage of the capacitor reaches the value of the reference voltage of the comparator, the comparator transmits a control pulse to the interval control unit. The control unit thereupon activates the screenwiper drive motor in a known manner by means of a square-wave pulse.
The control pulse of the comparator is simultaneously used as a discharge pulse for the capacitor. The wiping intervals become shorter, the more quickly the capacitor is recharged, i.e. the higher the electrical voltage generated by the sensor unit.
A basic frequency of approximately 3 intervals per minute is maintained, even without precipitations, in the "AUTOMATIC" position of the control switch. This frequency is controlled by a constant-current circuit connected in parallel to the sensor unit at the capacitor input. The vibratory plate is damped against high-frequency vibrations of the vehicle body by rubber elements.
The kinetic energy of striking precipitations is dependent on the mass of the drops and on the impact speed. In order to obtain the same wind conditions on the sensor unit as on the windscreen, it must be mounted in a region where a positive dynamic pressure prevails, even at a high driving speed.
The vehicle speed influences the impact intensity of precipitations, the SAE Paper mentioned above stating that the kinetic energy of precipitations is approximately dependent on the third power of the driving speed. A compensation for the vehicle speed is therefore necessary. In the known control, this is provided by a specific empirically determined angle of incidence of the vibratory plate of 15.degree. relative to the horizontal.
A system similar to that described above is also known from European Patent EP 0,115,338 A2.
Another device for controlling wiping intervals is known from German Patent Document DE 2,630,470 A1. This control uses a capacitive moisture sensor, the signals of which are intended to serve not only for detecting the density of striking precipitations, but also for determining the intensity of these precipitations. In this known control, the intensity of the precipitations is detected selectively by a differentiated surface structure of the sensor itself, which produces sensor output signals differing according to the different drop volumes or drop impact forces, or by an electrical evaluation of the levels of the sensor signals amplified in a pulse amplifier.
The purpose of such a detection of the intensity of striking precipitations is, in a sudden cloudburst, to obtain a quick reaction of the screenwiper drive and ensure a corresponding control of the spacing of the individual wiping cycles, that is to say of the wiping intervals.
The arrangements described above neglect an essential factor, namely the adaptation of the sensor systems to changes in the weight of individual particles of precipitation as a function of the temperature, especially around the freezing point. As is known, the weight of snowflakes is greater at temperatures above the freezing point ("wet snow") than at lower temperatures. Although a precipitation sensor of the above-described types responds reliably to "wet snow", because of their low weights, a similarly heavy fall of snowflakes at temperatures below freezing point does not load the sensor to such an extent that wiping operations can be triggered by means of the sensor's signal, even though the view of the vehicle driver is already being impeded, for example by snowflakes melting on the windscreen.
There is a known arrangement for controlling a screenwiper system (German Patent Document DE 3,538,553 A1), in which a moisture sensor is used and in which measures are taken to compensate the effects of fluctuations in the ambient temperature on the reliability of the arrangement. With these measures, which are merely intended to prevent a faulty triggering of wiping cycles at different temperatures of a measurement zone of the moisture sensor and of a reference measurement zone, it is not possible to obtain a controlled temperature-dependent change in the responsiveness of a sensor system having a pressure-sensitive sensor.
In a further known screen-cleaning device (German Patent Document DE 3,442,948 A1), the pressure force of the screenwipers can be varied by a pressure spring consisting of a material changing its spring rigidity and/or its geometry under temperature changes, in that the temperature of the pressure spring is adjusted according to the pressure force required at different vehicle speeds.
Neither of these documents provide a controlled temperature-dependent change in the responsiveness of a sensor system having a pressure-sensitive sensor.
An object of the invention is, therefore, to provide a method by which the sensitivity of a known sensor system to the particular driving and environmental conditions, especially to the ambient temperature, is improved, and to provide a device for carrying out this method.
This and other objects are achieved by the present invention which provides a method of adapting the sensitivity of a sensor system for the detection of precipitations during the control of wiping intervals of a screenwiper drive of a motor vehicle as a function of the kinetic energy of striking precipitations. The method comprises the steps of detecting striking precipitations with a pressure-sensitive sensor of a sensor system, converting the detections of striking precipitations into electrical output signals, transmitting the output signals via a transmission path to an amplifier with an amplification factor for activating a wiper-motor control circuit, and varying the electrical properties of the transmission path as a function of ambient temperature such that the responsiveness of the sensor system is increased with decreasing temperatures.
The objects described above are also achieved by a device constructed according to an embodiment of the present invention for controlling wiping intervals of a screenwiper drive of a motor vehicle. The device comprises a sensor system, this sensor system detecting striking precipitations and converting detected kinetic energy of the striking precipitations into electrical output signals. The sensor system includes a pressure-sensitive sensor, a transmission path coupled to the sensor and carrying the electrical output signals, and an amplifier coupled to the transmission path, this amplifier amplifying the electrical output signals. The device also includes a wiping-interval control unit coupled to the sensor system, an interval time of the wiping-interval control unit being variable by the sensor system as a function of the kinetic energy of striking precipitations. The invention includes means for changing the total electrical resistance of the transmission path between the sensor and the amplifier as a function of ambient temperature. In certain embodiments of the invention, the means for changing the total electrical resistance of the transmission path reduces the total electrical resistance of the transmission path with falling ambient temperatures and increases the total electrical resistance of the transmission path with rising ambient temperatures.
By the method according to the invention and the device for carrying it out, the wiping-interval function is improved and it becomes possible for the sensor system to differentiate between the various precipitated bodies. Especially during the processing of temperature and driving-speed information, the sensor system is adapted as closely as possible to the particular driving conditions. Thus, wiping operations of a motor-vehicle screenwiper system controlled by a sensor are initiated reliably even as a result of the impact of snowflakes at temperatures below the freezing point.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.